The present invention relates to a rare earth zeolite Y and the preparation process thereof
Catalytically cracked gasoline is a major component of the vehicle gasoline. Now, catalytically cracked gasoline in China accounts for about 70% of the vehicle gasoline. In recent years, as more and more attention of the whole society is paid to the environmental protection, it becomes more and more urgent to produce the low olefin, low sulfur, high quality clean catalytically cracked gasoline. However, the heavier feedstock for catalytic cracking has put forward higher requirements for the catalytic cracking catalyst, which should not only have higher activity, better selectivity and higher hydrogen transfer activity, but also have higher hydrothermal stability.
As the major component of the catalytic cracking catalyst, zeolite Y has gone through the following developing stages:
NaY is inactive for catalytic reactions, and only after being changed into HY by exchanging Naxe2x88x92 with NH4+, it possesses a pretty high activity. In 1960""s, zeolites have already been used in oil refining. The stability of zeolites is always an important problem which should be taken into consideration in the catalyst preparation, and especially in catalytic cracking. Zeolites must possess good hydrothermal stability.
U.S. Pat. No. 3,384,572, and U.S. Pat. No. 3,506,440 indicate that HY prepared by exchanging Na+ in zeolite with NH4+ has a pretty high activity but poor structure stability, which will lose its crystallinity in dry air at a temperature higher than 500xc2x0 C., and its structure will be destroyed even when placed in air at room temperature. Rare earth exchanged HY (abbreviated as REHY) prepared by further partly replacing NH4+ with rare earth ions based on HY has very high activity, and has also overcome the shortcoming of poor thermal stability of HY With the further development of catalytic cracking of heavy oil, although REHY and REY prepared by hybrid exchanging with rare earth ions and NH4+ have high hydrogen transfer activity and cracking activity, it also has the problems that its initial unit cell contracts difficultly and that it can not be stable under high hydrothermal conditions, which exhibits as easy to produce coke and deactivate in the catalytic cracking unit.
U.S. Pat. No. 3,293,192 and C. V. M C Darid and P. K. Maher, in xe2x80x9cZeolite stability and ultrastable zeolitexe2x80x9d (Zeolite Chemistry and Catalysis, Acs Monograph 171, Washington D.C., 1976, 285-231) described a procedure that NaY is exchanged with NH4+ in an aqueous solution and then the exchanged zeolite is calcined in steam at a temperature of 600xc2x0 C. to 825xc2x0 C., which can be carried out repeatedly. After such a treatment of multiple exchanging and calcining, the unit cells of the zeolite contract and achieve ultrastableness. Thus the ultrastable zeolite Y (abbreviated as USY) prepared by the hydrothermal process is obtained. Such kind of zeolite was widely used in 1990""s, which is characterized in the less unit cell contraction, good thermal and hydrothermal stability, low hydrogen transfer activity and good coke selectivity, but also has the disadvantage of the limited activity and needs to improve its activity by exchanging with a solution of rare earth. USY has a rather high silica to alumina ratio and less cation sites in its framework, meanwhile partial crystal strcture collapsed and pluged the pores during ultrastabilization, which results in the small ion exchange capacity of the USY. Generally, the content of RE2O3 in the rare earth exchanged USY (abbreviated as REUSY) is up to 3-4 wt %, and a considerable amount of RE2O3 is absorbed at the outside of zeolite crystal.
In summary, although REY and REHY have high contents of rare earth, namely 10-20 wt % of RE2O3 in REY and 6-14 wt % of RE2O3 in REHY, and have higher activity and hydrogen transfer activity, the unit cell can not further contract. The unit cell constant xcex10 is generally between 2.468-2.470 nm, the hydrothermal stability is relative poor, and the differential thermal collapsed temperature thereof is 900-980xc2x0 C. On the contrary, USY has a better unit cell contraction, namely xcex10=2.445-2.450 nm, a good hydrothermal stability, and the differential thermal collapsed temperature is higher than 1000xc2x0 C., but it contains very little rare earth because it is very difficult for the rare earth cations to enter into the lattice of the crystal. Besides, the unit cells of the above two types of zeolites contract in a large extent after aging, and the equilibrium unit cell constant is only 2.425-2.428 nm; therefore the hydrogen transfer activity is low and it is hard to meet the requirement of the new cracking catalyst, e.g. the selective hydrogen transfer activity.
Presently, a mixture of rare earth exchanged ultrastable zeolite Y, i.e. rare earth-containing ultrastable zeolite Y (the content of RE2O3 in REUSY is usually about 3 wt %), moderate rare earth content zeolite REHY (the content of RE2O3 is usually 6-14 wt %) and high rare earth content zeolite REY (the content of RE2O3 is usually 10-20 wt %) are combined and mixed in different ratios when a cracking catalyst is prepared. Although the activity of the catalyst is improved, due to the poor hydrothermal stability of REHY and REY per se, the thermal and hydrothermal stability of the catalyst become poor; therefore the catalyst easily deactivates in operation, which is reflected by the low activity of equilibrium catalyst and the poor coke selectivity. However, the amount of REUSY is increased from the conventional 30-35% to 40% or even 45%, there is no doubt that the production cost will be greatly raised.
It is an object of the present invention to provide a zeolite Y which simultaneously has high intracrystalline rare earth content, smaller initial unit cell constant, good thermal and hydrothermal stability, and can be directly used to prepare cracking catalysts. It is till another object to provide a process for preparing such a zeolite Y.
The content of the intracrystalline rare earth of the zeolite Y according to the present invention on the basis of RE2O3 is 4-15 wt %, the unit cell constant thereof is 2.450-2.458 nm, and the differential thermal collapsed temperature thereof is 1000-1056xc2x0 C.
The content of the intracrystalline rare earth of the zeolite Y according to the present invention on the basis of RE2O3 is preferably 6-12 wt %, and the unit cell constant thereof is preferably 2.452-2.456 nm. Such a zeolite has a higher framework silica to alumina ratio, which is 8.3-8.8, and a sodium oxide content less than 1.0 wt %, preferably 0.5 wt %.
The present invention also provides a process for preparing such a zeolite Y, which comprises: drying rare earth-containing zeolite Y as raw material so that the water content thereof is lower than 10 wt %, introducing gaseous silicon tetrachloride carried by dry air in a zeolite Y to silicon tetrachloride weight ratio of 1:0.1-0.9 and reacting at a temperature ranging from 150xc2x0 C. to 600xc2x0 C. for 10 min-6 b, then purging with dry air for 5 min-2 h, and washing with de-cationized water to remove the residual soluble by-products such as Naxe2x88x92, Clxe2x88x92, Al3+ and the is like in the zeolite.
In the provided process, said rare earth-containing zeolite Y can be either commercial products of REY and REHY, or the product derived from NaY by rare earth exchanging.
Generally, the content of the rare earth in said commercial product of REHY on the basis of RE2O3 is 6-14 wt %, and the content of Na2O is hither than 2 wt %; the content of the rare earth in said commercial product of REY on the basis of RE2O3 is 10-20 wt %, and the content of Na2O is higher than 4 wt %.
In the preparation process of zeolite Y, said procedure of the rare earth exchanging NaY is that NaY having a silica to alumina ratio higher than 3.5 is exchanged with an aqueous solution of rare earth chloride in a weight ratio of NaY:RECl3:H2O=1:0.1-0.25:5-15 under the conditions of pH greater than 3.5 and a temperature of 80-95xc2x0 C. for 30-60 min with or without drying to obtain the zeolite.
Said raw material REY, REHY, or NaY exchanged with a rare earth chloride aqueous solution should be dried before the reaction to ensure that the water content is less than 10 wt %, preferably less than 5 wt %.
Said temperature for the reaction of the zeolite and gaseous silicon tetrachloride carried by dry air may be 150-600xc2x0 C., preferably 200-500xc2x0 C.
The rare earth zeolite Y according to the present invention has: (1) good thermal and hydrothermal stability, e.g., the unit cell size remains well after being treated under severe conditions of 100% steam at a temperature of 800xc2x0 C. for 17 h; (2) good cracking activity and selectivity to heavy oils, selective hydrogen transfer activity and activity stability, good selectivity to coke, abilities to effectively improve the yield of light oils and improve the quality of gasoline, and a capacity to be directly used as an active component for preparing various hydrocarbon cracking catalysts.
The process for preparing the above rare earth zeolite Y according to the present invention makes full use of the characteristic of the gaseous silicon tetrachloride of easy diffusing into the pores of the zeolite, carries out the reaction of isomorphous substitution effectively, and realizes dealumination, silicon supplement, and sodium removal in one step without many times of exchange and calcination.
In addition, compared to the prior art described in CN 1127161A, the preparation process according to the present invention has wider raw material sources, which may be either REY and REHY or NaY; in particular when NaY is used as raw material, rare earth chloride does not need drying. The zeolite NaY is exchanged with the conventional aqueous solution of RECl3, making full use of the characteristic of zeolite NaY""s good adsorption and desorption, which significantly alleviate the uncertainty of the operation and severe preparation condition as to xe2x80x9cmixing zeolite NaY with the pulverized rare earth while they are hotxe2x80x9d taught in CN 1127161A.
The present invention will be further described below with examples, but is not limited thereto.
In the examples, the silica to alumina ratio is calculated by the following equation:                     SiO        2            /              Al        2              ⁢          O      3        =                    (                  2.5858          -                      a            0                          )                    (                              a            0                    -          2.4191                )              xc3x97    2  
Wherein xcex10 is the unit cell constant of zeolite measured by X-ray diffraction method.